What is the OSI model?

The Open Systems Interconnection (OSI) model is a conceptual model created by the International Organization for Standardization which enables diverse communication systems to communicate using standard protocols. In plain English, the OSI provides a standard for different computer systems to be able to communicate with each other.REF

The OSI Model viewed through three lenses

I have a workflow story to tell that I hope helps to explain the Open Systems Interconnection model (OSI). Designed in 1977, the OSI has been part of the ISO specifications since 1984 with each layer serving a unique task for network communication. See the standard here and here for reference.

To illustrate the story, I’ll use a scholar and mathematician named Paul Feehan who is about to submit his manuscript “Perturbations of local maxima and comparison principles for boundary-degenerate linear differential equations” electronically for publication in the Transactions of the American Mathematical Society (TRAN). Now, what follows is a more or less sequential telling of what happens in this process but in fact, much of what will happen will be so quick and seamless, it may seem like a parallel process.

I’ll break it out sequentially below so the full seven layers of the OSI model can be elucidated. In short though, when Paul’s computer requests the manuscript submission page (EditFlow) from the American Mathematical Society (AMS), his desktop machine will used an Ethernet, or wireless connection to relay information through a router, and the request will ultimately land on an HTTP server at the AMS where the page resides. Within a matter of a few seconds (depending on latency in the network), he will have seamlessly moved through all seven layers of the OSI stack.

In the following example, the OSI model was used from the upper-most layers to the lowest layers; from Layer 7 through Layer 1.

Layer 7

Application Layer

The OSI model defines the application layer as the user interface responsible for displaying received information to the user.

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Layer 6

Presentation Layer

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display.

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Layer 5

Session Layer

The session layer provides the mechanism for opening, closing and managing a session between end-user application processes

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Layer 4

Transport Layer

The transport layer provides services such as connection-oriented data stream support, reliability, flow control, and multiplexing.

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Layer 3

Network Layer

The network layer is responsible for packet forwarding including routing through intermediate routers

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Layer 2

Data Link Layer

The data link layer provides the functional and procedural means to transfer data between network entities

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Layer 1

Physical Layer

The physical layer consists of the basic networking hardware transmission technologies of a network.

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Reference

Open Systems Interconnection Model

The OSI Model was designed in 1977 and is part of the ISO specifications since 1984. Each OSI layer serves a unique task for network communication.

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Layer 7
Application Layer

Layer 7

The Standard

Abstract from the Standard- Section 7.2.2.3: The Presentation Layer ensures that the information content of the Application Layer data is preserved during transfer. Cooperating application-entities are responsible for determining the set of abstract syntaxes they employ in their communication. The Presentation Layer is informed of the abstract syntaxes that are to be employed. Knowing the set of abstract syntaxes to be used by the application-entities, the Presentation Layer is responsible for selecting mutually acceptable transfer syntaxes.

As I Understand It

The application layer is the top layer of the OSI model. It provides a set of interfaces for applications to obtain access to networked services as well as access to network services that support applications directly. This layer also provides application access security checking and information validation.

Feehan's Opus

In our case, Paul sits down at his desktop machine, fires up his browser and heads over to the early-2000s-like website for EditFlow – the AMS Manuscript Submission application.

At this point, he’s already touched several layers of the OSI stack, but we’ll just concentrate on this layer for now. At layer seven, and through his browser, Paul has access to file transfers, email, reading, writing, and deleting of files and management of remote file storage. This is where he’ll begin today’s work.

Layer 6
Presentation Layer

The Standard

Abstract from the Standard- Section 7.2.4: The Presentation Layer performs the following functions to help accomplish the presentation-services:

a) negotiation and re-negotiation of transfer syntax; b) representation of the abstract syntax chosen by the application-entities in the transfer syntax negotiated or renegotiated, including format and special purpose transformations (for example, data compression); c) restoration of previously negotiated syntaxes on the occurrence of certain events; and d) use of session-services.

As I Understand It

The presentation layer is responsible for the format of the data transferred during network communications. In the OSI model, the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For outgoing messages, it converts data into a generic format for the transmission. For the incoming messages, it converts the data from the generic form to a format understandable to the receiving application. A yin and yang process indeed. Different computers have different codes for representing data. The presentation layer makes it possible for computers with different representation to communicate. The presentation layer provides common communication services such as encryption, text compression, and reformatting.

Feehan's Opus

In Paul’s scenario, Layer 6 is going to allow his article to be placed into a format that can be accepted by the receiving application; in this case EditFlow. In short, this layer formats and encrypts data to be sent across a network – from his university to wherever it travels from there, and eventually to the servers at the American Mathematical Society.

More on the article’s path follows below.

Layer 5
Session Layer

The Standard

Abstract from the Standard- Section 7.3.3: In connection-mode, the services provided by the Session Layer are described here: a) session-connection establishment; b) session-connection release; c) normal data transfer; d) expedited data transfer; e) token management; f) session-connection synchronization; g) exception reporting; h) activity management; i) typed data transfer; and j) resynchronization.

As I Understand It

TThe session layer takes the stateless 6 and 7 layer and coordinates them into a cohesive unit that can now synchronize its data flow back and forth between layers. There are some who will allegorically describe the session layer as the manager of a catering team. The more people enter the dining hall, the greater need there us for unity amongst the workers in the front end of the house and back end of the house: Getting things ready, making sure everyone has what they need, making sure the place is cleaned up when the event is over, and turning out the lights on the way out the door.

Feehan's Opus

In the case of Paul’s paper, Layer 5 allows him to enter his information screen by screen into EditFlow which maintains this information into a cohesive whole. Once he reaches the final screen and can upload his paper and hit submit, Layer 5 has done its job in collecting data, error checking it, and making sure everything is “just so”, so that its next stop into Layer 4 will be smoothly orchestrated, and handed off cleanly.

Layer 4
Transport Layer

The Standard

Abstract from the Standard- Section 7.4.3: In connection-mode, the following facilities provided by the Transport Layer are described here: transport-connection establishment; transport-connection release; data transfer; expedited data transfer; and suspend facility.

As I Understand It

The transport layer provides Layers 5 through 7 a standardized access so they do not need to consider the characteristics of the communications network.

Feehan's Opus

So, what does that mean for Paul and his paper? Here’s where, after working his way through the EditFlow interface and attaching a PDF copy of his manuscript, Paul hits the submit button. The article works its way through Layer 4 where the data he wants sent is transferred to Layer 3 and his connection is released. The article is now on its way to being published.

Layer 3
Network Layer

The Standard

Abstract from the Standard- Section 7.5.3.1.3: The Network Layer contains functions necessary to provide the Transport Layer with a firm Network transport Layer boundary which is independent of the underlying communications media in all things other than quality of service. Thus the Network Layer contains functions necessary to mask the differences in the characteristics of different transmission and subnetwork technologies into a consistent network service.

As I Understand It

Layer 3, or the Network Layer, is commonly known as the layer where routing takes place. As we’ve learned throughout the course, a router's main job is to get packets from one network to another. Layer 3 protocols and technologies allow for network-to-network communications. In talking with the system administrators at the AMS, I learned that a great deal of what network administrators do happens at Layer 3: dealing with IP addresses and subnetting, routing protocols, firewall rules and Access Control Lists (ACLs), and many other types of services are enabled by Layer 3 technologies.

Feehan's Opus

Paul’s paper once released from Layer 4 has now entered the Network Layer. Here it is moving from Paul’s desktop at the Department of Mathematics at Rutgers University into the network where the layer’s task will be to take the packets of information that form Paul’s article and move them through the most efficient path it can find to deliver them to the AMS server that handles manuscript submissions.

Layer 2
Data Link Layer

The Standard

Abstract from the Standard- Section 7.6.2.1: The Data Link Layer provides functional and procedural means for connectionless-mode among network-entities, and for connection-mode for the establishment, maintenance, and release data-link-connections among network-entities and for the transfer of data-link-service-data-units. A data-link-connection is built upon one or several physical-connections. In connection-mode, the facilities provided by the Data Link Layer are: a) data-link-addresses; b) data-link-connection; c) data-link-service-data-units; d) data-link-connection-endpoint-identifiers; e) error notification; f) quality of service parameters; and g) reset.

As I Understand It

Layer 2, or the Data Link Layer, ensures that all packets of information are passed along free of errors amd makes sure the appropriate physical protocols are assigned to the data.

Feehan's Opus

Although he probably doesn't know it, Paul’s paper is benefiting greatly here through the layers of tasks of dealing with transmission errors, regulating the flow of data so the network isn't overwhelmed, and providing a well defined interface to the Network Layer. I've seen this layer referred to as the postmaster or postal service requiring that every letter or package meets certain requirements in order to move through the system and on to its next destination.

Layer 1
Physical Layer

The Standard

Abstract from the Standard- Section 7.7.2: The Physical Layer provides the mechanical, electrical, functional and procedural means to activate, maintain, and de-activate physical-connections for bit transmission between data-link-entities. A physical-connection may involve intermediate open systems, each relaying bit transmission within the Physical Layer. Physical Layer entities are interconnected by means of a physical medium.

As I Understand It

At the bottom of the OSI we have the Physical Layer, which represents the electrical and physical representation of the system. This includes everything from the cable type, radio frequency link (as in an 802.11 wireless system), as well as the layout of pins, voltages and other physical requirements. When a networking problem occurs, many tech support staff go immediately to the physical layer to check that cables are properly connected and the power plug hasn’t been pulled from the router, switch, or computer.

Feehan's Opus

In our scenario, after reviewing a few more items from MathSciNet, our author Paul Feehan has finally put the tinishing touches on the article. Without any fanfare, Paul just successfully interacted with the Physical Layer of the internet when sending requests through the hardware, Ethernet cable and other physical elements in his network setup.

All in all not a bad day for Paul and his newly submitted article